1. Field of the Invention
The present invention relates to a solid-state image pickup device. More specifically, the present invention relates to an amplifying-type solid-state image pickup device which provides high sensitivity by preventing a reduction in sensitivity due to the influence of junction capacitance of a photodiode and which exhibits low noise performance. The present invention also related to a solid-state image pickup device which can perform high accuracy cancellation of background light so as to provide an output signal associated with light projected from a light source.
2. Description of the Related Art
Various types of solid-state image pickup devices are known. These include amplifying-type solid-state image pickup devices such as an AMI, SIT, CMD, BASIS, FGA, BCMD, as well as those of MOS and CCD types. The amplifying-type solid-state image pickup devices can be used without any problems for applications of small-sized pixels. However, problems which will be described below occur for applications such as a sensor for use in autofocus control (AF) of a camera in which rather large pixels are required.
Taking AMI (Amplified MOS Imager)as an example, the problems will be described. FIG. 1 is a circuit diagram illustrating a common configuration of one pixel of an AMI, comprising a photodiode 1001, an amplifying transistor Q1, bias transistors Q2 and Q3, a transistor Q4 for a reset operation, a bias circuit 1002, and a switching transistor 1003 which is driven by an output pulse of a shift register. In the AMI configured in this manner, an output signal voltage .DELTA.V.sub.OUT obtained as a result of photoelectric conversion is given by EQU .DELTA.V.sub.OUT =I.sub.p .multidot.t/C.sub.d ( 1)
where I.sub.p is photocurrent, t is integral time, and C.sub.d is junction capacitance of the photodiode 1001.
As can be seen from equation 1, to obtain a greater output signal voltage .DELTA.V.sub.OUT while maintaining the integration period constant, it is required either to increase I.sub.p or to reduce C.sub.d. However, the increase in I.sub.p requires-an increase in the area of a pixel, which will result in an increase in C.sub.d. On the other hand, the reduction in C.sub.d requires an reduction in the area of a pixel, which will result in a reduction in I.sub.p. Therefore, it is impossible to increase the sensitivity of an AMI as long as the conventional configuration is employed.
One technique to solve the above-described problem is to employ a configuration disclosed in "A New MOS Imager Using Photodiode as Current Source" (IEEE Journal of Solid-State Circuits, Vol. 26, No. 8, Aug., 1991). In this configuration, transfer gate transistors Q5 and Q5 are added, and furthermore a storage capacitor C.sub.t is connected between the photodiode 1001 and the amplifying transistor Q1. In a solid-state image pickup device having such a pixel configuration, the transistor Q5 is turned on in response to a DATA signal so that the transistor Q5 operates in its saturation region during an integration period, thereby maintaining the voltage across the photodiode 1001 at a fixed value which is smaller by an amount equal to the gate-to-source voltage V.sub.GS than the gate voltage of the transistor Q5. As a result, photo-induced charges generated in the photodiode 1001 is stored via the transistor Q5 into the storage capacitor C.sub.t connected to the gate of the amplifying transistor Q1. In the above process, the influence of the junction capacitance C.sub.d of the photodiode 1001 is isolated, and thus the output signal voltage .DELTA.V.sub.OUT associated with the photoelectric conversion is given by EQU .DELTA.V.sub.OUT =I.sub.p .multidot.t/C.sub.t ( 2)
As can be seen from equation 2, if the capacitance of the storage capacitor C.sub.t is reduced, then the output signal voltage .DELTA.V.sub.OUT increases. This means that the sensitivity can be determined independently of the junction capacitance C.sub.d of the photodiode.
However, the solid-state image pickup device having the above-described configuration has a disadvantage which will be described below. If the amount of incident light is constant during the integration period, no problem occurs because the current (photo current I.sub.p) flowing through the transistor Q5 is constant. However, if the amount of the incident light changes during the integration period, the current flowing through the transistor Q5 also changes, which results in a change in V.sub.GS of the transistor Q5. As a result, charge transfer occurs between the junction capacitance C.sub.d of the photodiode and the storage capacitor C.sub.t, which gives rise to a problem that an accurate output cannot be obtained in the photoelectric conversion.
To solve the above problem, the inventor of the present invention has proposed a solid-state image pickup device having a pixel configuration shown in FIG. 3, which has been disclosed in Japanese Patent Application No. 4-36922 (1992). In FIG. 3, reference numeral 1011 denotes a photodiode, and reference numeral 1012 denotes an n-type MOS transistor wherein the source of the MOS transistor 1012 is grounded and its drain is connected to a p-type MOS transistor 1013 acting as a load so that a common-source type amplifier is formed. The input of this common-source amplifier, that is the gate of the n-type MOS transistor 1012, is connected to the photodiode 1011. The output of the common-source amplifier, that is the drain of the n-type MOS transistor 1012, is connected via a capacitor 1014 to the input terminal (the gate of the n-type MOS transistor 1012) so as to provide a feedback signal. Furthermore, an n-type MOS transistor 1015 is connected in parallel to the capacitor 1014 so that the n-type MOS transistor 1015 functions as a switching transistor for resetting the gate voltage of the n-type MOS transistor 1012 to an initial potential. A solid-state image pickup device is formed with a one- or two-dimensional array of photoelectric conversion detection cells (pixels) each having the above-described configuration, wherein there is provided a n-type MOS transistor 1016 which is driven by a shift register pulse to select a pixel to be read out in such a manner that when the n-type MOS transistor 1016 is turned on, a drain voltage of the n-type MOS transistor 1012 appears on the signal output line 1017.
In the photoelectric conversion detection cell having the above configuration, photo-induced charges generated in the photodiode 1011 are stored into the capacitor 1014. Therefore, if the capacitance C.sub.t of the capacitor 1014 is reduced and the gain of the common-source amplifier is increased sufficiently, then the sensitivity increases. The bias current flowing through the n-type MOS transistor 1012 is set to a sufficiently large value compared to the photo-induced current so that the voltage across the photodiode 1011 may be maintained constant, whereby a correct photoelectric conversion output can be obtained regardless of a change in the photo-induced current.
However, the solid-state image pickup device having the configuration shown in FIG. 3 has the following problem. If the capacitance C.sub.t of the capacitor 1014 is reduced so as to obtain a higher sensitivity, the noise generated by the n-type MOS transistor 1012 is amplified by a ratio of C.sub.d /C.sub.t. As a result, the effect of the noise generated by the n-type MOS transistor 1012 becomes so large that it cannot be neglected. Therefore, something should be done to reduce the noise generated by the n-type MOS transistor 1012.
In general, noise of an MOS transistor consists of thermal noise and 1/f noise. The thermal noise is proportional to (L/W).sup.1/2, and the 1/f noise is inversely proportional to L.multidot.W where L is the channel length of the MOS transistor and W is the channel width. Therefore, if the channel length L is reduced to an attainable minimum length and the channel width is increased sufficiently enough, then the noise is reduced.
However, if the channel width is set to a large value, the parasitic capacitance between the gate and the drain of the n-type MOS transistor 1012 in the pixel configuration shown in FIG. 3 becomes large, which gives rise to an effect equivalent to an increase in capacitance C.sub.t of the capacitor 1014, which thus results in a reduction in the sensitivity.